Add support for importing GL_R16F for YUVA promise images

Change-Id: Id9bfc98e7154dd41f3954c5824a3ab04afce3f50
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206900
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

src/gpu/gl/GrGLCaps.cpp

@@ -3242,6 +3242,9 @@ static GrPixelConfig get_yuva_config(GrGLenum format) {
         case GR_GL_RGB10_A2:
             config = kRGBA_1010102_GrPixelConfig;
             break;
+        case GR_GL_R16F:
+            config = kAlpha_half_as_Red_GrPixelConfig;
+            break;
     }
 
     return config;